The present invention relates to a method for the flash butt welding of annular workpieces, such as chain links in which the ends of the workpiece that are to be welded to each other, are first preheated in several successive preheat time cycles and then are welded together by subsequent flashing and upsetting.
In flash butt welding by means of direct or alternating current, the preheating is used to apply a sufficient amount of heat, depth of heat, and welding temperature to the two workpiece ends to be welded together for subsequent welding by flashing and upsetting. The heat energy applied to the workpiece ends during the preheat time cycles can be expressed by I.sup.2 .times.R.times.t where
I=welding current PA1 R=resistance of workpiece including the transfer resistance between the two workpiece ends PA1 T=total preheating time.
The welding current is given by the secondary voltage of the welding transformer stage, the impedance of the welding machine, the weld cross-section, and the chemical composition of the workpiece to be welded. The total preheating time is determined by the weld cross-section, the welding current density, and the chemical composition of the workpiece to be welded. Hence, optimization of the preheating process can be achieved only via the variable transfer resistance between the workpiece ends which must be made as large as possible to achieve rapid heating.
Based on the relation that the transfer resistance between the workpiece ends to be welded is increased and the heating of the workpiece ends is speeded up, the smaller the contact force between the workpiece ends during the preheat time cycles, in order to optimize the preheating operation, then minimum contact force is used between the workpiece ends in case of stretched workpieces (open lengths). The contact force as difference between feeding force and friction of the upsetting slide becomes smaller as the feeding force is reduced. As a rule, one selects a feeding force which is about 20% above the force required to overcome the friction.
With annular workpieces, among which are chain links, a force to deform the workpiece rear is required in addition to achieve the required contact force and to overcome the carriage friction. At the present time it is customary to make the feeding force for annular workpieces from the very outset so high, that sufficient feed of the upsetting slide is ensured during the entire preheating operation, and dispensing in such a case with the desired minimum contact force during the initial preheat time cycles.
Based on this state of the art, it is the object of the present invention to develop a method which provides optimization of the preheating process for annular workpieces to achieve maximum speed of preheating of the workpiece ends to be welded.
Another object of the present invention is to provide a method of the foregoing character which is simple to apply and may be used economically.
A further object of the present invention is to provide a method, as described, in which the welded ends have a substantially long service life.